I personally think it’s safer to enforce data constraints in application code, because “bad” data won’t break it. Putting the constraints in the database means your application is coupled to whatever database you use. Worse, because databases only offer a small set of constraints, it’s often the case that constraints are split between the database and application code. You have “validations” in both the application code and database definitions, and data must be checked/transformed anyway whenever it is marshaled to and from the database. Constraints in the database are effectively implicit, living in a set of migrations or often not part of any source control.

EAV indexing is much easier, because EAV is naturally represented as a tree. The whole database is the index. You only have four universal indexes that cover every query: EAV, AEV, AVE, and VAE. The mutual likes example above is covered by these indexes. And you never had to worry about it!

See Datomic for real world example of an EAV architecture.